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TPS62120 Questions regarding the datasheet.

Andrejs Gasilovs
Expert 1335 points
Other Parts Discussed in Thread: TPS62120

Hello,

I'm currently designing a board where I plan to use TPS62120. I have been examining the datasheet and I have some questions regarding it:

1) In table 7.5 current I_LIMF is defined only for V_in = 8V. I wonder how it changes depending on the input voltage? Are the values for 3.3V, 8V or 12V much different?

2) In equation (2) shouldn't there be [A] instead of [mA]?

3) I calculated C_ff according to equation (7) for R1=560k and obtained the value 11pF that is two times smaller than the one proposed in the table. I know it doesn't matter much, but is there a reason to go with 22uF?

4) In the Power Supply and Layout section there is nothing regarding bulk capacitor. Won't it hurt much if I will add a 100uF electrolytic capacitor before the input capacitor (main power source is couple of meters away)?

Excuse me for kind a pedantic questions, but I want to be very sure about each detail. Thanks in advance.

  • 0
    TI__Guru*** 148100 points
    Sure, you can ask questions here. What Vin, Vout, and Iout do you need?

    1) Yes, all parameters can vary over Vin. The 8Vin just shows the single point where this spec is measured. Current limit usually increases with increasing Vin. The device is designed to always deliver at least its 75 mA rated current.
    2) Yes, this should be in A, if you use the usec and uH inputs
    3) Either value is likely ok. The Cff is usually tuned to your specific application. Having more Cout downstream is one reason for this.
    4) Yes, it is a good idea to add a bulk Cin in your case. This can go anywhere but its effectiveness increases as it gets closer to the IC. Here is a blog I wrote on this very topic: e2e.ti.com/.../what-is-that-giant-tantalum-cap-on-the-input-of-the-evm
  • 0 in reply to Chris Glaser
    Expert 1335 points
    Chris,

    Thank you for your answer.
    My configuration is: Vin = 12V, Vout = 3.3V, Iout_max < 75 mA.
    I have yet a couple of questions:

    1) If I'm not planning to use PG (POWER GOOD) pin, is it ok to leave it floating or is it still beneficial to tie it to Vout with a pullup resistor?
    2) Datasheet says that for inductor one needs to have "quality factor above 25 at the switching frequency". Mostly this graph is not provided by the vendors, typically they specify some Q value on some test frequency. For instance, for the recommended Coilcraft LPS3015 I was unable to find any info regarding Q factor at all. I have found a couple of good inductors with Q=17 specified on f = 3 MHz. Will they do the job?
  • 0 in reply to Andrejs Gasilovs
    TI__Guru*** 148100 points
    Yes, you can leave PG floating.

    The Q factor is mainly for ensuring low AC losses which keeps a high efficiency at light loads. More important is the inductance and saturation current rating.
  • 0 in reply to Chris Glaser
    Expert 1335 points

    Thank you Chris.

    It is amazing that one can get a help from experts so fast. Cheers :)